Inkjet printers form printed images using one or more printheads, each one of which includes an array of inkjet ejectors. A controller in the printer operates the ejectors to form printed images that often include both text and graphics and may be formed using one or more ink colors. Some inkjet printers receive and carry print media, such as paper sheets, envelopes, or any other article suitable for receiving printed images, on a drum past one or more printheads to receive the ink drops that form the printed image. Many printers that use drums to transport print media include a vacuum plenum and holes in the drum to generate a suction force through the surface of the drum. Each print medium engages a portion of the holes on the surface of the drum and the suction force holds the print medium to the surface of the drum to prevent the print media from slipping or otherwise moving relative to the surface of the drum as the drum rotates the media past the printheads. Holding each print medium in place relative to the surface of the moving drum enables the printer to control the timing of the operation of printheads to ensure that the printheads form printed images in proper locations on each print medium and ensures that the print media do not cause jams or other mechanical issues with the printer. In large-scale printer configurations, the drum can carry a plurality of print media simultaneously.
One problem with drums that carry print media over a vacuum plenum is that the print media often do not completely cover every hole in the drum. For example, as a drum carries two or more print media, a gap between sheets of consecutive print media can include holes exposed to the vacuum plenum. The relative locations of gaps on the drum often change between print jobs that use print media of different sizes. The suction force of the vacuum plenum draws air through the exposed holes near the edges of the print media, which produces airflow. In regions around the printheads, the airflow can affect the paths of ink drops as the ink drops travel from the printhead to the surface of the print medium, which can reduce the accuracy of drop placement and degrade image quality, particularly near the leading and trailing edges of the print media. For example, FIG. 6 depicts printed images produced by a prior art printer where text printed near a trailing edge of a document exhibits degraded image quality due to the airflow near the printhead. The upper character is a character located on one side of the trailing edge of a medium sheet carried by a drum, the middle character is located in the center of the medium sheet, and the lower character is located at the opposite edge of the medium sheet. The air disruption discussed below with regard to FIG. 5 explains the scattered ink in the characters.
FIG. 5 illustrates the airflow that produces the degraded image quality shown in FIG. 6. FIG. 5 is a side-view schematic diagram illustrating a portion of a printing device 420. The printhead 424 is supported in a frame 414 along with a baseplate 416. Media sheets 428 are carried by a drum surface 418, shown as a portion in the figure, with a gap between the trailing edge of the rightmost sheet 428 and the leftmost sheet 428 as the sheets travel in the direction indicated by the arrow. The drum has holes and a vacuum source positioned below the drum surface 418 pulls air above the sheets against the sheets to hold the sheets to the drum. A gap between the frame 414 and the baseplate 416 enables air to follow path 422 through the gap between the printhead 424 and the trailing edge of the rightmost sheet 428 into the hole 126 as the printhead is ejecting ink onto the trailing edge. This air can displace the ink being ejected toward the trailing edge of the media sheet and produce the results shown in FIG. 6. A similar airflow produces similar results in ink ejected towards the leading edge of the next sheet. Thus, for printing systems that use a vacuum beneath a drum transport to hold media against the drum as the media pass the printheads, the areas between sheets produce a disruptive airflow. This airflow causes turbulence in the area between the printheads and the media sheets that deflects ink droplets from their intended trajectory. Consequently, improved media drum transport systems that provide suction force to hold print media in place while reducing or eliminating the negative effects of airflow due to exposed holes near printheads in the printer would be beneficial.